Modelling tropical forest responses to drought and El Niño with a stomatal optimization model based on xylem hydraulics

Abstract

The current generation of dynamic global vegetation models (DGVMs) lacks a mechanistic representation of vegetation responses to soil drought, impairing their ability to accurately predict Earth system responses to future climate scenarios and climatic anomalies, such as El Niño events. We propose a simple numerical approach to model plant responses to drought coupling stomatal optimality theory and plant hydraulics that can be used in dynamic global vegetation models (DGVMs). The model is validated against stand-scale forest transpiration (E) observations from a long-term soil drought experiment and used to predict the response of three Amazonian forest sites to climatic anomalies during the twentieth century. We show that our stomatal optimization model produces realistic stomatal responses to environmental conditions and can accurately simulate how tropical forest E responds to seasonal, and even long-term soil drought. Our model predicts a stronger cumulative effect of climatic anomalies in Amazon forest sites exposed to soil drought during El Niño years than can be captured by alternative empirical drought representation schemes. The contrasting responses between our model and empirical drought factors highlight the utility of hydraulically-based stomatal optimization models to represent vegetation responses to drought and climatic anomalies in DGVMs.This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.

Hydraulic traits explain differential responses of Amazonian forests to the 2015 El Niño‐induced drought

“…New Phytologist with the decrease in G s , which is likely maintaining ET constant despite changing VPD, as predicted by stomatal optimization models (Sperry & Love, 2015;Eller et al, 2018). We also show that xylem embolism resistance explained part of the response in leaf Ψ during the 2015-ENSO (Fig.…”

“…This highlights the role of xylem embolism resistance traits in determining plant functioning and vegetation drought response (Anderegg et al ., , ). Actually, including embolism resistance in plant models has improved the prediction of ecosystem transpiration drought responses in the Amazon forest of Caxiuanã (Eller et al ., ), and such models also predict lower sensitivity to drought than previous models, a result supported by our data.…”

Coupling of El Niño events and long-term warming leads to pervasive climate extremes in the terrestrial tropics

“…The pronounced increases in VPD are of particular importance because of vegetation's sensitivity to high VPD that causes declines in transpiration (Cowan andFarquhar 1977, Law et al 2002). Vegetation transpiration and productivity are often more sensitive to VPD than soil moisture deficits (Novick et al 2016), because leaf transpiration is understood to be highly sensitive to increases in leaf-to-air VPD (Wolf et al 2016, Eller et al 2018. The positive anomalies of shortwave radiation and temperature only serve to further augment the leaf-to-air VPD gradient, and by extension, limit the capacity of vegetation to absorb CO 2 for photosynthesis.…”

A theoretical and empirical assessment of stomatal optimization modeling

“…1b). Other models use different penalty criteria, often explicitly incorporating the risk of vascular damage (Wolf et al , 2016; Sperry et al , 2017; Anderegg et al , 2018; Eller et al , 2018) and/or NSL (Givnish & Vermeij, 1976; Hölttä et al , 2017; Dewar et al , 2018).…”